Burner provided with means against the overheating of the burner head

ABSTRACT

A burner ( 2; 2   a   ; 11; 25; 31; 34 ), comprising a burner body ( 12; 26; 32; 35 ), wherein a mixture of fuel and air can be introduced, and a burner head ( 14; 27; 32; 35   a   ; 41 ), wherein a plurality of openings ( 15; 28; 40; 44 ) is provided, thr ough which said mixture can flow, is associated with shield means ( 3; 16; 29: 33; 36; 42 ) shielding said burner head ( 14; 27; 32; 35   a   ; 41 ) suitable for limiting the maximum temperature that said burner head ( 14; 27; 32; 35   a   ; 41 ) can reach during working of the burner ( 2; 2   a   ; 11; 25; 31; 34 ).

[0001] The invention relates to a burner provided with protection means against burner head overheating.

[0002] A burner, in particular a burner fed with gaseous fuel, consists of a metallic body wherein a mixture of air and fuel is introduced; in a portion of said body, called head, slots are provided from which said mixture flows and is led to combustion.

[0003] The burner body, during working, is exposed to thermal stresses depending on the burner maximum working temperature and on the thermal gradient to which the burner is exposed. Excessive thermal stresses may remarkably reduce burner lifetime.

[0004] When a burner is installed into a boiler having high specific power, the temperature of the combustion chamber, in particular of its walls, reaches very high values. This causes a severe heat irradiation towards the burner housed into the combustion chamber, which produces negative effects such as burner body overheating, leading to a shortening of burner lifetime, and increase in temperature of the mixture introduced into the burner body, leading to a decrease in air density in the mixture and, then, decrease in air mass in the mixture per unity mass of fuel. This produces an increase in NOx production during combustion and consequently an increase in polluting emissions of the burner. Moreover, the increase in mixture temperature causes an increase in mixture combustion rate, which further increases the burner surface overheating.

[0005] Furthermore, in natural suction burners wherein, in full power working conditions, i.e. in nominal power working conditions, the mixture of air and gaseous fuel introduced in the burner is of hyperstoichiometric type, another disadvantage occurs. When said natural suction burners are working in reduced power conditions, i.e. in conditions of power lower than nominal power, the air dragging capability by the gas jet introduced in the burner is remarkably reduced. This reduction in dragging capability by the gas implies a reduction in mixture aeration rate. When the aeration rate approaches the stoichiometric value, the mixture combustion takes place very close to the burner head surface which, therefore, warms up much more than in nominal power working conditions, wherein the mixture combustion takes place at a certain distance from the burner head surface. Such overheating produces mechanical stresses due to structural constraints in the burner head. Said mechanical stresses, combined with the decreased mechanical strength of the burner head material, due to overheating, can produce, in a short time, mechanical damages of burner head, and can even lead to burner head failure. In this case, the whole burner needs to be replaced.

[0006] In known prior art, attempts have been made in order to obviate the above-mentioned overheating phenomena by cooling the burner head by means of internal ducts, into which cooling water is forced to circulate. This solution shows various disadvantages. First of all, the burner head cooling system implies remarkable additional costs; furthermore, the water flowing into cooling ducts produces, as time passes, a deposit build-up which strongly reduces the system cooling capability. This involves the risk that local burner head overheating can occur, with consequent structural damages and noise, due to the water flow into ducts clogged by said deposits and/or to resonances due to a change in the resonant frequency of the burner.

[0007] In certain conditions, for example in minimum power working conditions, the boiler draught causes an excessive primary air suction, which implies poor combustion and excessive CO emissions. In such conditions, cooling the burner head worsens the phenomenon of the excessive primary air suction.

[0008] Furthermore, cooling the burner head causes, in certain minimum power working conditions, the suction of an excessive amount of primary air, which implies an increase in CO emissions due to partial mixture combustion.

[0009] Other solutions known from prior art make use, in order to obviate burner head overheating, of materials having high temperature resistance for burner head production, said materials having substantially a tissue type structure and consistency. These solutions imply a very high cost and, furthermore, they do not allow natural suction burners to be produced, due to high pressure drops caused by the used material structure.

[0010] The present invention intends to provide means for controlling temperature effects on a burner head and for controlling the phenomenon of the excessive primary air suction, said means being simple, effective and cheap.

[0011] According to the present invention, a natural suction burner is provided, comprising a burner body wherein a mixture of fuel and air can be introduced, a burner head associated with said body and provided with a plurality of openings through which said mixture can flow, characterized in that it comprises shield means shielding said burner head, suitable for limiting the maximum temperature that said burner head can reach during working of the burner.

[0012] Said shield means can be so positioned as to shield the surface of said burner from irradiation of heat coming from environment surrounding the burner, for example from the walls of a combustion chamber into which the burner is inserted.

[0013] In such way, the above-mentioned shield means allow heat transfer by irradiation from the combustion chamber walls to the burner body surface to be avoided, or at least reduced, since said shield means are interposed between said walls and the burner surface, so as to absorb, and partially reflect, the heat emitted by irradiation by said walls towards the burner surface.

[0014] The shield means can consist of a further burner head, provided with further openings through which said mixture can flow, said further burner head being superimposed to said burner head and being connected thereto through connecting means which allow thermal expansions of said further burner head with respect to said burner head.

[0015] The further head, being capable of expanding freely, can withstand high thermal stresses without undergoing excessively high mechanical stresses, i.e. mechanical stresses capable of damaging it. Moreover, the further head acts as a protection shield of the burner head against said high thermal stresses, preventing so the burner head from reaching excessive temperatures.

[0016] In such way, the burner head lifetime is remarkably improved. The invention will be better understood and carried out from the following description, merely made for exemplifying and not restrictive purpose, with reference to the enclosed drawings, wherein:

[0017]FIG. 1 is a schematic, partial section of the combustion chamber of a boiler, wherein a plurality of burners and a first version of shield means according to the invention are installed;

[0018]FIG. 2 is a top view of one of the shield means shown in FIG. 1;

[0019]FIG. 3 is a left view of FIG. 2;

[0020]FIG. 3a is a transverse section of a first different embodiment of the shield means of FIG. 2;

[0021]FIG. 3b is a transverse section of a second different embodiment of the shield means of FIG. 2;

[0022]FIG. 4 is a schematic, exploded view of a tubular burner with a second version of shield means according to the invention;

[0023]FIG. 5 is a longitudinal section of the burner head of FIG. 4;

[0024]FIG. 6 is a schematic view of a burner head provided with a third version of shield means according to the invention;

[0025]FIG. 7 is a longitudinal section of the burner head of FIG. 6;

[0026]FIG. 8 is a perspective view of a burner of the so-called “modular blade” type, provided with a fourth version of shield means according to the invention;

[0027]FIG. 9 is a top view of FIG. 8;

[0028]FIG. 10 is the section X-X of FIG. 9;

[0029]FIG. 11 is a schematic view of a burner head provided with a fifth version of shield means according to the invention;

[0030]FIG. 12 is the section XII-XII of FIG. 11;

[0031]FIG. 13 is a perspective, schematic view of a burner provided with a sixth version of shield means according to the invention;

[0032]FIG. 14 is a longitudinal section of the FIG. 13;

[0033]FIG. 15 is a perspective, schematic view of a burner provided with a seventh version of shield means according to the invention;

[0034]FIG. 16 is a view in elevation of the burner of FIG. 15;

[0035]FIG. 17 is a perspective, schematic view of a burner head provided with an eighth version of shield means according to the invention;

[0036]FIG. 18 is a longitudinal section of the burner head of FIG. 17;

[0037]FIGS. 19 and 20 show a different embodiment of the burner of FIGS. 13 and 14.

[0038]FIG. 1 schematically shows a combustion chamber of a boiler, a wall 1 thereof is indicated and wherein a plurality of burners 2, 2 a is installed. A plurality of shield means 3 is arranged between each couple of adjacent burners 2 and between each burner 2 a, adjacent to a wall 1 of the combustion chamber, and the respective wall 1. The shield means consists of flat elements 3 having a major dimension oriented parallel to the longitudinal axis of burners 2. The flat elements 3 are arranged at the upper portion 5 of each burner 2, wherein slots are provided for the flow and the subsequent combustion of the mixture of air and fuel, by which each burner 2 is fed. Flat elements 3 are vertically positioned so as to be below the ignition plane of flames produced by said mixture combustion. The flat elements 3 extend, preferably, along the whole combustion chamber length, so as to shield a body surface of the burners 2 as large as possible from the heat irradiated by the walls 1 of the combustion chamber. Preferably, the flat elements 3 are not physically in contact with the body of the burners 2, in order to prevent heat from being transmitted by conduction to the burners 2. In any case, if points of physical contact between flat elements 3 and burners 2 are needed in order to support said flat elements 3, these points must be minimized, so as to reduce heat transfer by conduction from the flat elements 3 to the burners 2. Preferably, each flat element 3 is supported in an isostatic manner, for example by the combustion chamber structure. For instance, each flat element 3 can be rigidly fixed, at a first end, to the combustion chamber structure and, at a second end opposite to said first end, it can be fixed to the combustion chamber structure so as to be free to move in direction of said major dimension, in order to counterbalance the thermal expansion to which it is exposed. Each flat element 3 can be furthermore provided with at least one longitudinal stiffening rib 4, in order to increase its bending stiffness.

[0039] Any material resisting to high temperatures, to remarkable thermal stresses and having mechanical properties allowing thin walled plates, with high bending and/or torsional stiffness to be obtained, can be used for flat elements 3 production.

[0040] It is possible to use, for example, refractory steel, ceramic refractory material, or cermet.

[0041] The flat elements 3, arranged as explained before, are placed on the path of thermal radiating energy emitted by the walls of the combustion chamber 1 and directed towards the burners 2. Therefore, the flat elements 3 shield the burners 2, preventing them from directly receiving, on the major portion of their surface, the radiant energy emitted by the walls 1. This causes a reduction in the maximum temperature that can be reached by the bodies of burners 2 during standard operation, which increases lifetime of burners 2 and decreases NOx content in combustion smokes. If circulation of secondary air, or of other gaseous fluid, is needed between the burners 2, said flat elements 3 can be provided with suitable openings 51 (FIGS. 3a and 3 b) allowing said circulation, without compromising the shielding of burners 2. For example, the openings can be provided with side shieldings 52 (FIG. 3a) or 53 (FIG. 3b), suitable for preventing heat irradiated by said walls 1 from reaching the burners 2 passing through the holes 51.

[0042] In FIG. 4 a tubular burner 11 is shown, provided with a second version of shield means according to the invention. The burner 11 is provided with a body 12 comprising at its upper portion a head 13, which is connected to the burner body, or which is integral portion thereof. In a region 14 of the head 13, constituting the diffuser of burner 11, openings 15 are provided, for instance shaped as slots, through which a mixture of air and fuel, fed in the burner body 12, flows. A further head 16, constituting the above-mentioned shield means, is superimposed to the head 13 of the burner 11. The further head 16 is provided with further openings 17, for instance shaped as slots, through which said mixture flows.

[0043] The dimensions of the slots 15 and of the further slots 17 and the position of the further head 16 with respect to the head 13 are chosen so that the slots 15 and the further slots 17 are at least partially superimposed, so as to define a flowing section for the discharge, and subsequent combustion, of the mixture of air and fuel fed into the body 12 of the burner.

[0044] The further head 16 is fixed to the head 13 of the burner through fixing means which allows the further head 16, that is positioned closer to the ignition region of said mixture combustion and is therefore exposed to the highest working temperatures, to thermally expand with respect to the head 13 without producing in the further head 16 excessive mechanical stresses, i.e. stresses which could damage the further head 16. The fixing means is of a substantially removable type so as to allow the further head 16 to be easily separated from the head 13, without damaging the latter. In such manner, if the further head 16 needs to be replaced, it is possible to do that without replacing the remainder of the burner. The fixing means can consist of spot welding S, preferably made in a central region of the further head 16. It is advantageous that the welding spots are reduced to a minimum number, preferably one. In such manner, the further head 16 is constrained to the head 13 only at this point and is therefore free to thermally expand with respect to the head 13, without that significant mechanical stresses are generated due to this expansion. Alternatively to the spot welding S, it is possible to use any connecting means suitable for connecting in a removable manner the further head 16 to the head 13 of the burner in a restricted region of its surface, such as, for instance, a screw, a rivet, etc., so arranged as to allow the largest freedom of relative movement between the head 13 and the further head 16.

[0045] In order to keep the further head 16 properly positioned with respect to the head 13, guide means is provided, comprising, for example, a couple of rivets 18, 19 fixed to the head 13 and suitable to be coupled.with clearance with a pair of notches, 20, 21 provided in the end regions of the further head 16. In this manner, the further head 16 remains properly positioned with respect to the head 13 and, therefore, with respect to the diffuser 14, even when it expands as a result of thermal stresses to which it is exposed during the working of the burner 11.

[0046] The dimensions of the slots 15 and of the further slots 17 and the position of the further head 16 with respect to the head 13 are chosen so that, in full power working conditions, the further slots 17 and the slots 15 are mutually aligned so as to define the largest flowing section to the mixture of air and fuel with which the burner is fed. The slot dimensions are selected so that, in this condition, the pressure drops due to the mixture passage through the slots 15 and the further slots 17 are substantially equal to the pressure drops of a traditional burner. To do that, it is necessary to compensate the pressure drops, which are higher than the ones existing in a traditional burner, due to the longer extension of the mixture flowing channels through the head 13 and the further head 16. This compensation can be obtained, for example, by increasing the overall flowing area for the mixture, with respect to a traditional burner, or by choosing slot section shapes which can reduce the pressure drops.

[0047] If it is necessary to obviate the disadvantage of an excessive boiler draught, in minimum power or near minimum power working conditions, the dimensions of the slots 15 and of the further slots 17 and the position of the further head 16 with respect to the head 13 can be advantageously chosen so that, in such working conditions, the flowing section for the mixture of air and fuel is throttled. The amount of air sucked by the burner can thus be reduced, and an excess of primary air can thus be avoided, which would cause an increase in CO emissions.

[0048] The dimensions of the slots 15 and of the further slots 17 and the position of the further head 16 with respect to the head 13 can also be chosen so that the overall flowing section for the mixture of air and fuel is substantially constant in any normal working conditions of the burner.

[0049] If the burner 11 works in conditions in which the mixture combustion starts near the surface of the further head 16, said further head 16 acts as a shield for the head 13, preventing it from overheating and therefore protecting the integrity of the burner body 12.

[0050] In FIG. 6 a tubular burner is shown, provided with a third version of shield means according to the invention. Also in this third version, the shield means consists of a further head 16, connected to the head .13 through connecting means comprising a “U” shaped small plate 22. Said small plate 22 passes through a first opening 23, made, for example, in an end region of the head 13 and through a corresponding second opening 24, which is superimposed to said first opening 23 and which is made in a corresponding end region of the further head 16. The small plate 22 is mounted by pressure so as to mutually constrain the head 13 and the further head 16 at said end region.

[0051] The end of the further head 16 opposite to the one in which the second slot 23 is provided is free, so as to allow the further head 16 to expand with respect to the head 13, without undergoing mechanical stresses of dangerous value. The position of said small plate 22 can be chosen so as to maximize mutual sliding between head 13 and further head 16 in the most suitable position for the superimposition of slots 15 and further slots 17.

[0052] In FIG. 8 a burner 25 is shown of the so-called “modular blade” type, provided with a body 26, on the upper portion of which a burner head 27 is fixed. In the burner head 27 openings 28 are provided, for instance shaped as slots, through which a mixture of fuel and air introduced in the burner body 26 can flow. The region of the head 27 wherein the slots 28 are provided constitutes the diffuser of the burner 25.

[0053] A further head 29 is superimposed to the head 27, said further head 29 being provided with further openings 30, for instance shaped as slots, through which said mixture can flow. The further head 29 constitutes a fourth version of shield means according to the invention. The dimensions and the position of the slots 28 and of the further slots 30 and the position of the further head 29 follow the previously explained rules, with reference to the second version of shield means, shown in FIGS. 4 and 5. The further head 29 is fixed to the head 27 in a pre-established position, in a way similar to that previously described with reference to the burner shown in FIGS. 4 and 5. For instance, a first end of the head 29 can be fixed at 29 a to the head 27, while another end of said further head 29, opposite to said first end, is free to slide with respect to the head 27 when expanding due to thermal stresses to which it is exposed in working conditions.

[0054] In FIGS. 11 and 12 a burner 31 of radiant type is shown, provided with a fifth version of shield means according to the a portion of which a set of openings 32 a is provided, for instance shaped as slots, for the flow of a mixture of fuel and air with which the burner is fed. A second tubular body 33 is applied over the tubular body 32, said second tubular body 33 being provided with further openings 33 a, for example shaped as slots, through which the mixture flowing through the slots 32 a of the first tubular body 32 can discharge.

[0055] The second tubular body 33 constitutes shield means according to the invention. The second tubular body 33 is coupled to the first tubular body 32 so as to be axially slidable with respect to it, in order to be able to freely expand, while working, with respect to the first tubular body 32, due to thermal stresses to which it is exposed. Again, the dimensions and the position of the slots 32 a and of the further slots 33 a and the position of the second tubular body 33 follow the previously explained rules, with reference to the second version of shield means, described in FIGS. 4 and 5.

[0056] In FIGS. 13 and 14 a burner 34 is shown, provided with a sixth version of shield means according to the invention. The burner 34 comprises a tubular body 35, the rear portion of which comprises a burner head 35a wherein openings 40 are provided, for instance shaped as slots, for the flow of a mixture of a fuel and air with which the burner 34 is fed. A further head 36 is superimposed to the head 35 a, said further head 36 being also provided with openings 37, for instance shaped as slots, for the flow of said mixture, after it has passed through the slots 40 of the head 35 a.

[0057] Said further head 36 constitutes shield means according to the invention.

[0058] The further head 36 is fixed to the body 35 through a couple of fixing elements 38 shaped as brackets, fixed to the body 35 through welding spots or any other fixing means, for example screws, allowing an easy removal of the fixing elements 38 without damaging the body 35, whenever it is necessary to replace the further head 36. The further head 36 is furthermore fixed, preferably in a central position, at 36 a, to the body 35 through welding spots, or any other means suitable to assure a simple and reliable removal of the further head 36 from the body 35. The brackets 38 are so shaped and arranged that the further head 36, due to thermal variations to which it is exposed while working, can freely expand without undergoing remarkable mechanical stresses due to said thermal variations. The further head 36 is so arranged as the slots 37, provided thereon, are substantially aligned with the slots 40 of the head 35 a.

[0059] In FIGS. 17 and 18 a different embodiment of shield means illustrated in FIGS. 13 and 14 is shown. In this embodiment, the further head 36 is connected to the body 35 of the burner through fixing elements 39 shaped as straps, fixed to the body 35 similarly to the brackets 38. Also the straps 39 are so arranged as to allow the further head 36 to freely expand with respect to the body 35 of the burner, without undergoing remarkable mechanical stresses.

[0060] In FIGS. 19 and 20 a still further version of a burner according to the invention is shown, wherein the burner 50 is of a type with substantially flat head 41, to which a further head 42 is superimposed, said further head 42 being also substantially flat. Both the head 41 and the further head 42 are provided with openings 44 and 45, respectively, for instance shaped as slots, for the flow of a mixture of fuel and air with which the burner is fed.

[0061] The further head 42 is connected to the head 41 through connecting elements 43, for example “C” shaped, arranged along a couple of opposite sides of the first head 41 and of the further head 42. The connecting elements 43 are so arranged as the further head 42 can freely expand with respect to the head 41, without undergoing remarkable mechanical stresses. The two connecting elements 43 are furthermore connected to the burner body so as to be easily removable, if the head 41 or the further head 42 has to be replaced.

[0062] The dimensions and the position of the slots 44 and 45 and the position of the further head 42 follow the previously explained rules, with reference to the second version of shield means, shown in FIGS. 4 and 5.

[0063] In FIGS. 15 and 16 a variant is shown of the burner illustrated in FIGS. 13 and 14, wherein the further head 36 is provided with end ribs 46 and 47 and with a middle rib 48, arranged transversely to the further head 36 and substantially perpendicular thereto. The presence of ribs 46, 47 and 48 allows the fixing elements 38 to be eliminated and the further head 36 to be fixed to the burner body 35 through spot welding, made, for example, at the middle rib 48. This is possible since the ribs 46, 47 and 48 constitute stiffening elements for the further head 36, and prevent its ends from rising when the further head 36 warms up. Furthermore, the ribs 46, 47 and 48 assure a more uniform heating of the further head 36, reducing thus the mechanical stresses due to differential heating of said further head 36.

[0064] In the practical implementation, the materials, the dimensions and the constructive details can be different from those indicated, but technically equivalent thereto, without exiting therefore from the legal domain of the present invention. 

1. Burner (2; 2 a; 11; 25; 31; 34), comprising a burner body (12; 26; 32; 35) wherein a mixture of fuel and air can be introduced, a burner head (14; 27; 32; 35 a; 41) wherein a plurality of openings (15; 28; 40; 44) is provided, through which said mixture can flow, characterized in that it is associated with shield means (3; 16; 29; 33; 36; 42) shielding said burner head (14; 27; 32; 35; 41) suitable for limiting the maximum temperature that said burner head (14; 27; 32; 35 a; 41) can reach during working of the burner (2; 2 a; 11; 25; 31; 34):
 2. Burner according to claim 1, wherein said shield means (3) is so positioned as to shield said burner head from the heat irradiation originating from walls (1) of a combustion chamber in which said burner (2; 2 a) can be inserted.
 3. Burner according to one of the preceding claims, wherein said shield means (3) is positioned below the ignition plane of flames produced by combustion of said mixture.
 4. Burner according to one of the preceding claims, wherein said shield means comprises at least one flat element (3).
 5. Burner according to claim 4, wherein said at least one flat element (3) has a major dimension oriented in a direction substantially parallel to a longitudinal axis of said burner (2; 2 a).
 6. Burner according to claim 4, or 5, wherein said at least one flat element (3) is supported in an isostatic manner.
 7. Burner according to claim 6, wherein said at least one flat element (3) is rigidly supported at a first end by a structure element of said combustion chamber.
 8. Burner according to claim 6, or 7, wherein said at least one flat element (3) is supported, at a second end opposite to said first end, by another structure element of said combustion chamber, so as to be slidable in a direction parallel to said major dimension.
 9. Burner according to one of claims 4 to 8, wherein said at least one flat element (3) is provided with at least one stiffening rib (4) extending in a direction substantially parallel to said major dimension.
 10. Burner according to one of claims 4 to 9, wherein said at least one flat element (3) is provided with through openings (52).
 11. Burner according to claim 10, wherein said through openings (52) are associated to further shield means (52; 53).
 12. Burner according to one of the preceding claims, wherein said shield means (3) is made of a material resisting to high temperatures and to remarkable thermal stresses.
 13. Burner according to one of the preceding claims, wherein said shield means (3) is made of a material having high bending and/or torsional stability.
 14. Burner according to one of the preceding claims, wherein said shield means (3) is made of a material chosen from a group comprising refractory steel, refractory ceramic material and cermet.
 15. Burner according to claim 1, wherein said shield means comprises a further burner head (16; 29; 33; 36; 42), which can be superimposed to said burner head (14; 27; 32; 35 a; 41) and fixed thereto so as to be capable of sliding with respect to said burner head (14; 27; 32; 35 a; 41) as an effect of thermal expansions.
 16. Burner according to claim 15, wherein said further burner head (16; 29; 33; 36; 42) comprises a plurality of further openings (17; 30; 37; 45) through which said mixture can flow.
 17. Burner according to claim 15, or 16, wherein said further openings (17; 30; 37; 45) can be aligned with said openings (15; 28; 40; 44).
 18. Burner according to one of claims 15 to 17, wherein said further burner head (16; 29; 33; 36; 42) is connected to said burner head (14; 27; 32; 35 a; 41) through connecting means (S; 12; 19 a; 26 a; 28; 29; 33) substantially removable.
 19. Burner according to one of claims 15 to 18, wherein said burner head (14) is provided with first guide means (19) for said further burner head (16), said first guide means (19) being suitable for coupling with second guide means (20, 21) provided in said further burner head (16).
 20. Burner according to claim 19, wherein said first guide means comprises pin means (19).
 21. Burner according to claim 19, or 20, wherein said second guide means comprises slot means (20, 21).
 22. Burner according to one of claims 18 to 21, wherein said connecting means comprises “U” shaped small plate means (22).
 23. Burner according to claim 22, wherein said “U” shaped small plate means (22) is insertable in a first opening (23) provided in said burner head (14) and in a respective second opening (24) provided in said further burner head (16).
 24. Burner according to one of claims 18 to 21, wherein said connecting means comprises bracket means (38).
 25. Burner according to claim 24, wherein said bracket means (38) is fixed in a substantially removable manner to said burner head (35 a), near the ends of said further burner head (36).
 26. Burner according to one of claims 15 to 25, wherein said further burner head (36) is provided with ribs (46, 47, 48).
 27. Burner according to claim 26, wherein said ribs (46, 47, 48) extend transversely to said further burner head (16) and in a direction substantially perpendicular to the surface of said further burner head (16).
 28. Burner according to claim 26, or 27, wherein said ribs (46, 47, 48) comprise a first rib (46) arranged at a first end of said further burner head (16) and a second rib (47) arranged at a second end of said further burner head (16).
 29. Burner according to one of claims 26 to 28, wherein said ribs (46, 47, 48) comprise a further rib (48) arranged in a substantially central position of said further burner head (16).
 30. Burner according to one of claims 18 to 21, wherein said connecting means comprises strap means (39).
 31. Burner according to claim 30, wherein said strap means (39) is fixed in a substantially removable manner to said head, near the ends of said further burner head (36).
 32. Burner according to one of claims 18 to 21, wherein said burner head is a portion of a first tubular body (32) and said further burner head is a portion of a second tubular body (33), substantially externally concentric to said first tubular body (32).
 33. Burner according to one of claims 18 to 21, wherein said burner head (41) and said further burner head (42) are substantially flat.
 34. Burner according to claim 33, wherein said burner head (41) and said further burner head (42) are mutually connected through connecting elements (43) arranged along a couple of respective opposite sides of said burner head (41) and of said further burner head (42).
 35. Burner according to claim 34, wherein said connecting elements (43) have a “C” shaped cross section.
 36. Burner according to claim 34, or 35, wherein said connecting elements (4-3) are connected in a removable manner to the burner body.
 37. Burner according to one of claims 16 to 36, wherein the dimensions of said openings (15; 28; 40; 44) and of said further openings (17; 30; 37; 45) and the position of said further burner head (16; 29; 33; 36; 42) with respect to said burner head (14; 27; 32; 35 a; 41) are so chosen that, in full power working conditions, the further openings (17; 30; 37; 45) and the openings (15; 28; 40; 44) are mutually aligned so as to define the largest flowing section for said mixture.
 38. Burner according to one of claims 16 to 37, wherein the dimensions of said openings (15; 28; 40; 44) and of said further openings (17; 30; 37; 45) and the position of said further burner head (16; 29; 33; 36; 42) with respect to said burner head (14; 27; 32; 35 a; 41) are so chosen that, in minimum power or near minimum power working conditions, the flowing section for said mixture of air and fuel is throttled.
 39. Burner according to one of claims 16 to 36, wherein the dimensions of said openings (15; 28; 40; 44) and of said further openings (17; 30; 37; 45) and the position of said further burner head (16; 29; 33; 36; 42) with respect to said burner head (14; 27; 32; 35 a; 41) are so chosen that the overall flowing section for said mixture of air and fuel is substantially constant in any normal working conditions. 